In order to obtain correct wiping of the swept surface, it is necessary that the wiping strip of the wiper blade should be applied on the swept surface with a substantial pressing or wiping force. To this end at least one spring is usually provided, for example a helical tension spring which is disposed between the drive head and the remainder of the wiper arm, and which applies to the wiper arm a wiping torque the nominal value of which is substantially constant, so as to urge the wiper blade strip against the swept surface.
With this design of wiper, the wiping strip is in contact with the swept surface not only during the actual wiping movement, but also when the wiper is in its rest or parked position. Due to the fact that the nominal value of the wiping force which is applied to the wiping strip when the wiper is at rest is constant, it is found that the wiping strip retains a residual deformation resulting from the flexing of its profile against the swept surface, so that the wiping effect obtained from the wiping strip is not effective enough. It is therefore desirable to be able to reduce the value of the wiping force which is applied to the wiping strip when the wiper itself is at rest.
In addition, the quality of the wiping of the swept surface depends in operation on proper control of the wiping force which is applied to the wiping strip, especially having regard to the travelling speed of the vehicle, the frequency of the wiping movement of the wiper itself, and the position of the wiping strip on the windshield.
In this connection, and having regard to the curvature of the swept surface, the wiping strip is not applied with a substantially constant wiping force over its whole length in the various positions which it occupies during one wiping cycle. It is therefore desirable to be able to guarantee that the wiping strip is applied with a sufficiently large wiping force against the swept surface in all its various positions during the wiping operation.
Finally, in the course of one wiping cycle, it is also desirable to be able to reduce the wiping force at the two extreme positions occupied by the wiper during the cycle, in order, especially, to facilitate reversal of its motion.